In engine systems with split exhaust manifold, a blowdown exhaust valve of a cylinder may be opened first to deliver exhaust mass flow from an initial portion of an exhaust phase to a turbine of a turbocharger or a turbine-driven generator, while a scavenging valve may be opened later to deliver exhaust mass flow from a latter portion of the exhaust phase directly to an exhaust catalyst, bypassing the turbine. In this way, by directing exhaust gases away from the turbine during the latter portion of the exhaust phase, the pumping penalty associated with high turbine backpressure may be reduced.
One example of such a split exhaust engine system is illustrated by Robel in U.S. Pat. No. 8,091,357. Therein, an exhaust system includes a turbo compounding device located in a first exhaust branch, and an exhaust gas treatment device is located in a second exhaust branch. The turbo compounding device receives exhaust gases through a first exhaust valve and the exhaust gas treatment device receives exhaust gases through a second exhaust valve. A timing strategy for controlling the first exhaust valve and the second exhaust valve is based on a pressure sensor utilized to monitor an exhaust backpressure.
However, the inventors herein have recognized potential issues with such a system. As one example, Robel does not provide any method for controlling a turbine speed and/or an output of the turbo compounding device. For example, during engine operation, a turbine speed may increase above a threshold which when left unchecked may lead to sub-optimal and unsafe performance of the system.
Thus in one example, some of these issues may be at least partly addressed by a method for an engine, comprising: in response to a turbine speed greater than a first threshold turbine speed, retarding an opening time of a first exhaust valve of a cylinder delivering exhaust gas to a turbine of the turbine-generator. In this way, by adjusting timing of the exhaust valve, an amount of exhaust mass flow to the turbine may be reduced.
For example, a split exhaust engine system may include a first exhaust valve (herein referred to as blowdown valve) for delivering a first portion of exhaust energy (herein referred to as blowdown energy) to a turbine of a turbine-generator located in a first exhaust passage. The engine system may further include a second exhaust valve (herein referred to as scavenging valve) for delivering a latter portion of exhaust energy (herein referred to as scavenging energy) to an exhaust catalyst located in a second, different exhaust passage. The catalyst may be located downstream of the turbine-generator such that output of the turbine-generator passes through the catalyst. During engine operating conditions when a turbine speed of the turbine-generator is greater than a threshold speed or when a generator output is greater than a threshold output, an engine controller may reduce an amount of exhaust gas (that is, blowdown gas) delivered to the turbine by retarding an opening timing of the blowdown valve; and/or advancing an opening timing of the scavenging valve and a closing timing of the scavenging valve. In some examples, a duration of the scavenging valve may be increased by advancing the opening timing of the scavenging valve with little or no advance of closing timing of the scavenging valve. Further, a degree of retard and a degree of advance of the blowdown and the scavenging valve may be adjusted based on one or more of the turbine speed, a difference between a desired turbine speed and an actual turbine speed, a difference between a desired generator output and an actual generator output, and an engine speed/load condition.
In some other examples, in addition to adjusting the timing and the duration of the blowdown and the scavenging valves, the opening degree of a wastegate valve may be adjusted to change an amount of exhaust gas bypassing the turbine.
In this way, by modulating phasing and duration of the blowdown and the exhaust valves based on turbine speed and/or generator output to reduce the amount of exhaust energy delivered to the turbine of the turbine-generator, the occurrence of turbine over-speed and excess generator output conditions which may decrease an efficiency and/or durability of the engine system may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.